Run off is the water that drains out the bottom. You water to run off to mainlu get the roots at the bottom wet and drain out the salt build up wheb using liquid nutes
i dont PH run off so i can't really tell you what a good % is for run off but my best guess is 10-15% at least im running an experiment for myself thats why where you get 0 run off so far it seems to work okay. you can read into it as im not too familiar with the technique but basically it's so it doesn't leech the nutrients too much supposedly good for super soils that need to feed the plants throughout the entire grow for the most part